Transformation of the buffer layer grown on 4H-SiC to single-layer graphene by ex situ hydrogen intercalation

Abstract

The buffer layer samples grown on the Si-face of the 4H- and 6H-SiC substrates were heated in a hydrogen flow at different temperatures (600–900?°C) and heating times (40–60?min) in order to obtain quasi-freestanding monolayer graphene. Their structural properties were characterized before and after heating by using the methods of Raman spectroscopy, atomic force microscopy, Kelvin probe force microscopy, and low-energy electron diffraction. The dependence of the degree of coverage of the sample with quasi-freestanding graphene and the number of defects in the resulting films on the heating temperature was studied. As a result of optimization of the technological parameters, it is shown that the highest quality of the resulting quasi-freestanding graphene can be achieved by using the following parameters: heating time of 40?minutes and temperature of 800?°C.     

Investigation of the structure of (p)3C-SiC-(n)6H-SiC heterojunctions

The structure of (p)3C-SiC-(n)6H-SiC epitaxial structures obtained by sublimation epitaxy in vacuum on 6H-SiC substrates was studied by methods of X-ray topography, X-ray diffraction, and transmission electron microscopy. The results showed high structure perfection in the epitaxial layers of both SiC polytypes with a sharp interface between the 3C-SiC and 6H-SiC layers.     

Fabrication of quasi-superlattices at the interface between 3C-SiC epitaxial layer and substrates of hexagonal SiC polytypes by sublimation epitaxy in vacuum

Transmission electron microscopy has been used to study the structure of a transition layer between a hexagonal substrate (6H-SiC and 4H-SiC) and a cubic silicon carbide layer grown by sublimation epitaxy in vacuum. It is shown by microdiffraction analysis that the transition layer with a thickness of 210 nm is constituted by alternating layers of cubic (3C) and hexagonal (6H) silicon carbide. It is demonstrated that 6H-SiC/3C-SiC and 4H-SiC/3C-SiC quasi-superlattices can be produced by this method.     

Development of a spinodal decomposition model for the example of a heterostructure based on silicon carbide polytypes

The transition region of a 3C-SiC/4H-SiC heterostructure constituted by layers of the 3C and 4H polytypes has been studied. A previously proposed spinodal decomposition model was used to estimate the thickness ratio of 4H and 3C layers in comparison with the image furnished by transmission electron microscopy.     

拉曼面扫描表征氮掺杂6H-SiC晶体多型分布

采用物理气相传输法(Physical Vapor Transport, PVT)沿[0001]方向生长了一个直径为2英寸的氮掺杂6H-SiC晶体. 采用拉曼面扫描方法对晶体中多型的分布进行了细致表征, 研究了SiC晶体生长过程中多型的产生和演化. 在6H-SiC晶体中观察到了15R-SiC和4H-SiC两种多型. 在拉曼面扫描得到的晶体多型分布图上观察到了两类次多型结构区域, 一类是继承其生长界面上对应的次多型结构形成的次多型结构区; 另一类是由温度、压力等生长条件波动导致在6H-SiC主多型中出现的15R-SiC多型结构区. 第一类次多型结构区中掺入的氮元素较多, 载流子浓度较高, 并且随着晶体生长不断扩大; 第二类次多型结构区对晶体结晶质量的影响较小, 且提高生长温度可以抑制15R-SiC多型结构.     

Fine Structure of the Blue Photoluminescence in High-Purity Hexagonal GaN Films

The photoluminescence of GaN films grown by molecular-beam epitaxy on 6H-SiC(0001) substrates was studied. Owing to the high structural perfection of the films, a series of narrow emission lines related to donor–acceptor recombination was detected in the blue spectral region. This finding lends support to the donor–acceptor origin of the blue emission in GaN. In the spectra of less perfect films, these lines overlap and merge into a broad band.     

MicroRaman spectroscopy of protective coatings deposited onto C/C–SiC composites

Abstract

MicroRaman spectroscopy has been used in the present work to investigate the structure and composition of pyrolytic carbon (PyC) and SiC protective coatings formed under various chemical vapour deposition conditions. Analysis of spectra obtained during Raman line mapping experiments on samples with graded Si_xC_y layer in the region of about 700–1000 cm^?1 allows information to be extracted on different SiC polytypes. It was found that the graded SiC layered sample contained a mixture of 3C–SiC and 6H–SiC polytypes at the film/substrate interface, but for the major part of this layer the 3C–SiC (β-SiC) phase predominates. For pure PyC films, it was found that the formation of PyC layer begins at 1200°C and the layer formed at this temperature is more uniform with slightly larger crystallite size (～3 nm) compared to that in the layer formed at 1300°C.     

MBE-growth of heteropolytypic low-dimensional structures of SiC

The controlled growth of SiC heteropolytypic structures consisting of hexagonal (-) and cubic (3C-) polytypes has been performed by solid-source molecular beam epitaxy. On on-axis substrates, 4H/3C/4H-SiC(0001) and 6H/3C/6H-SiC(0001) structures were obtained by first growing some nanometers thick 3C-SiC layer at lower temperatures (1550 K) and Si-rich conditions, and subsequent growth of -SiC on top of the 3C-SiC layer at higher temperatures (1600 K) under more C-rich conditions. On off-axis substrates, multi-heterostructures consisting of 4H/3C- or 6H/3C-stacking sequences were also obtained by first nucleating selectively wire-like 3C-SiC nuclei on the terraces of well-prepared off-axis -SiC(0001) substrates at low T (<1500 K). Next, SiC was grown further in a step-flow growth mode at higher T and Si-rich condition. After the growth many wire-like regions consisting of 3C-SiC were found within the hexagonal SiC layer material matrix indicating a simultaneous step-flow growth of both the cubic and the hexagonal SiC material.     

Gaps in the spectrum of epitaxial graphene formed on silicon carbide polytypes

Silicon carbide NH-SiC polytypes with N = 2, 4, 6, and 8 are considered as substrates for the epitaxial formation of graphene. The density of states for the substrates is described using the Haldane-Anderson model. It is shown that this model always leads to the appearance of two gaps in the graphene spectrum, which are adjacent to the valence and conduction bands of the substrate. The gap widths are determined by the ratio of the energy of interatomic interaction in the free-standing graphene sheet and the energy of graphene-substrate interaction. If this ratio is very small, the gap widths may increase so as to jointly cover almost the entire bandgap of the substrate; on the contrary, if this ratio is extremely large, both gaps exhibit narrowing and become negligibly small.     

Si(111)衬底上6H-SiC薄膜的低压化学气相外延生长与表征

在Si(111)衬底上,采用SiH4-C3H8-H2气体反应体系,通过低压化学气相沉积(LPCVD)工艺外延出结晶质量良好的SiC薄膜.低温光致发光谱表明该薄膜属于6H-SiC多型体.X射线衍射图表明该薄膜具有高度的择优取向性.扫描电子显微镜图表明该薄膜由片状SiC晶粒组成.拉曼光谱和透射电子衍射谱的结果进一步表明该薄膜具有较高的结晶质量.对Si(111)衬底上6H-SiC薄膜的生长机制进行了初步探讨.     

Hetero-epitaxy and structure characterization of Si films on 6H-SiC substrates

In order to realize the non-ultraviolet application of SiC optoelectronic devices, Si/6H-SiC heterojunctions were prepared by the low-pressure chemical vapour deposition at 850 °C. The X-ray diffraction (XRD) and the selected area electron diffraction (SAED) results indicate that Si thin films have a monocrystalline structure and were grown along the (111) crystal plane. The rationality of the (111) growth plane was also analyzed by the theoretical calculation. High-density structural defects such as stacking faults and twins were observed on Si films by the high-resolution transmission electron microscopy. This phenomenon was also validated by the SAED patterns of defect-rich regions on Si films.     

Microsyntaxy and polytypism in SiC

An interesting type of syntaxy has been observed by transmission electron microscopy where two different polytypes of SiC, 6H and 15R in one case and 6H and 4H in another case, coexist as alternating parallel bands of $\text{200～500}\text{A}\text{?}$ in width with the common c-axis. It is pointed out that this phenomenon is very closely related to an extensive formation of twin bands during the growth of β-SiC and to the coexistence of two kinds of martensite of close packed structure as alternating parallel bands in the transformation products of β-phase noble metal base alloys.     

Electrodeposition of graphene layers doped with Brϕnsted acids

A simple and fast method is demonstrated for the preparation of a thin film of graphene layers by the electrodeposition of positively doped graphene dispersion onto desired electrode substrates. A thin film of graphene layers was obtained by applying negative potentials according to the electrophoretic deposition mechanism. The doped graphene dispersion was prepared from expanded graphite treatment with various acids (HCl, HNO₃, and H₂SO₄) and an ultrasonication process. The doping and deposition processes are strongly dependent on the type of acid and the applied potential, which were monitored by Raman spectroscopy and quartz crystal microbalance, respectively. The morphology and electrochemical properties of the graphene film were characterized by scanning electron microscopy and cyclic voltammetry. The electrochemical performance of graphene film obtained using nitric acid or hydrochloric acid dopant is superior to that obtained with sulfuric acid doping. This technique could be a facile tool for the fabrication of a thin film of graphene layers on a desired substrate.     

Thermal stability of LiF thin films on 6H-SiC(0001) surface

The morphology and the temperature induced changes of LiF thin layers deposited on three different kinds of 6H-SiC(0001) surfaces have been investigated by the atomic force microscopy technique. As the substrates used were: a single crystal of Si-terminated on-axis oriented additionally hydrogen-etched, off oriented 3.5° from basal plane and with an epitaxial adlayer grown on the Si- or on the C-face surface. For all the systems investigated, independently of the substrate, the LiF grows uniformly at room temperature, and even for rather thick films (estimated to be about 5 nm), terraces are still visible. However, in each case the LiF layers are not temperature stable, and when the samples are heated (starting from ∼600 K), regular grains with radius up to 40 nm and with height up to 9 nm are formed (for initial layers thickness 2.5 nm). Subsequent sample heating procedures cause LiF desorption and consequently, a decrease of the dimensions of the islands.     

Excimer laser ablation of single crystal 4H-SiC and 6H-SiC wafers

A 248 nm, 23 ns pulsed excimer laser was used to compare the ablation characteristics of single crystal wafers of the polytypes 4H-SiC and 6H-SiC over a wide range of energy fluence (0.8–25 J cm⁻²). Photothermal models based on Beer–Lambert equation using thermal diffusivity and absorption coefficient, energy balance, and heat transfer were presented to predict the ablation mechanisms. Micromachining of trenches was demonstrated at 7 J cm⁻² to demonstrate the potential of UV laser ablation. Results indicate that the ablation process is characterized by two well-defined threshold fluences: (a) decomposition threshold ~1 J cm⁻² and (b) melting threshold ~1.5 J cm⁻² for both polytypes. Contrary to the modeling expectations, the ablation rates were lower and did not increase rapidly with energy fluence. Four types of ablation mechanisms—chemical decomposition, vaporization, explosive boiling, and plasma shielding—either singly or in combination occur as a function of energy fluence. The predictions of photothermal models were not in good agreement with the experimental data implying that a complex interplay among various physical phenomena occurs during ablation. Micromachined trench exhibited ripple patterns, microcracks and recast layers, most of which could be eliminated by a subsequent chemical cleaning process. It is concluded that excimer laser ablation is an effective but slow material removal process for SiC wafers compared to other lasers such as 1064 nm Nd:YAG.     

Thin-film PZT/SiC structure on silicon substrate: Formation,structural features,and dielectric properties

A new thin-film structure representing the Pt/PZT/SiC/Si system is obtained. The structure comprises a lead zirconate titanate Pb(Zr,Ti)O₃ (PZT) film deposited onto thin (90–100 nm) single crystal silicon carbide layers of 3C-SiC and 4H-SiC polytypes grown by a new solid-phase epitaxy on single crystal silicon substrates. Methods used for the formation of this multilayer structure are described, and its structural and dielectric characteristics are presented.     

Homoepitaxial 6H-SiC thin films by vapor-liquid-solid mechanism

Silicon carbide (SiC) is a IV-IV compound semiconductor with a wide energy band gap. Because of its outstanding properties, SiC can be used in high-power, high-temperature devices with high radiation resistance. In this study, a two-step vapor-liquid-solid (VLS) method was proposed for homoepitaxial growth of high quality 6H-SiC thin films, combining VLS growth and conventional chemical vapor deposition (CVD) processes. VLS growth was used to eliminate the micro-pipes (MPs) in the first step, and the subsequent step based on the CVD process was employed to improve the surface roughness. The morphology and structure of the as-grown thin films were investigated by scanning electron microscopy, X-ray energy dispersive spectroscopy, atomic force microscopy and high-resolution X-ray diffraction, showing that thin films grown by two-step method have good crystalline quality and small surface roughness.     

Nanocomposite for methanol oxidation: synthesis and characterization of cubic Pt nanoparticles on graphene sheets

Abstract

We present our recent results on Pt nanoparticles on graphene sheets (Pt-NPs/G), a nanocomposite prepared with microwave assistance in ionic liquid 2-hydroxyethanaminiumformate. Preparation of Pt-NPs/G was achieved without the addition of extra reductant such as hydrazine or ethylene glycol. The Pt nanoparticles on graphene have a cubic-like shape (about 60 wt% Pt loading, Pt-NPs/G) and the particle size is 6 ± 3 nm from transmission electron microscopy results. Electrochemical cyclic voltammetry studies in 0.5 M aqueous H₂SO₄ were performed using Pt-NPs/G and separately, for comparison, using a commercially available electrocatalyst (60 wt% Pt loading, Pt/C). The electrochemical surface ratio of Pt-NPs/G to Pt/C is 0.745. The results of a methanol oxidation reaction (MOR) in 0.5 M aqueous H₂SO₄ + 1.0 M methanol for the two samples are presented. The MOR results show that the ratios of the current density of oxidation (I_f) to the current density of reduction (I_b) are 3.49 (Pt-NPs/G) and 1.37 (Pt/C), respectively, with a preference by 2.55 times favoring Pt-NPs/G. That is, the tolerance CO poisoning of Pt-NPs/G is better than that of commercial Pt/C.     

A study of the effect of electron and proton irradiation on 4H-SiC device structures

The changes of the current–voltage characteristics and the uncompensated donor-impurity concentration (N _d –N _a ) in the base electrode of Schottky diodes and JBS diodes based on 4H-SiC have been studied upon their irradiation with 0.9-MeV electrons and 15-MeV protons. The carrier-removal rate was 0.07–0.15 cm^–1 under electron irradiation and 50–70 cm^–1 under proton irradiation. It was shown that the current–voltage characteristics of the devices under study remain rectifying at electron irradiation doses of up to ~1017 cm^–2. It was demonstrated that the radiation hardness of the SiC-based devices under study substantially exceeds that of silicon p–i–n diodes with similar breakdown voltages.

     

Catalyst-free growth of nanographene films on various substrates

We have developed a new method to grow uniform graphene films directly on various substrates, such as insulators, semiconductors, and even metals, without using any catalyst. The growth was carried out using a remote plasma enhancement chemical vapor deposition (r-PECVD) system at relatively low temperatures, enabling the deposition of graphene films up to 4-inch wafer scale. Scanning tunneling microscopy (STM) confirmed that the films are made up of nanocrystalline graphene particles of tens of nanometers in lateral size. The growth mechanism for the nanographene is analogous to that for diamond grown by PECVD methods, in spite of sp2 carbon atoms being formed in the case of graphene rather than sp3 carbon atoms as in diamond. This growth approach is simple, low-cost, and scalable, and might have potential applications in fields such as thin film resistors, gas sensors, electrode materials, and transparent conductive films.